A photoelectric encoder generally uses a bundle fiber, which is a bundle of fibers fixed to one another. A photoelectric encoder of related art includes a scale, a light emission fiber, a plurality of light reception fibers, and a detection head. The scale has a diffraction grating formed at a predetermined pitch in the direction of the measurement axis. The detection head, which can change its relative position with respect to the scale, irradiates the scale with coherent light. Light reflected by the scale enters the detection head. The outgoing coherent light propagates through the light emission fiber. The incoming reflected light propagates through the light reception fibers. The light emission fiber is disposed at the center on a detection plane of the detection head. The light reception fibers are disposed around the light emission fiber on the detection plane. The system of related art further includes and an interface unit that includes a light source, a light-sensitive element, and a signal processing unit. The light source supplies a laser beam to the light emission fiber in the detection head. The light-sensitive element receives the reflected light propagating through the light reception fibers and converts the received light into an electric signal. The signal processing unit processes the signal to measure a relative positional shift due to the movement of the detection head, the scale, or both. Examples of the related art are disclosed in the following Japanese Unexamined Patent Application Publications: Nos. 2004-53605, 2008-39602, and 2007-232681.
However, to carry out remote measurement by using such a photoelectric encoder of related art with a certain distance between the detection head and the interface (e.g., 20 m or longer), it is necessary to extend the plurality of fibers. Therefore, it is necessary to manufacture a long bundle fiber, which is not easy. It is hard to say that a photoelectric encoder of related art offers easiness in handling, usability, and work efficiency satisfactorily. Moreover, in the photoelectric encoder of related art, it is necessary to insert the bundle fiber into the interface and branch the plurality of fibers into the light emission fiber and the light reception fibers. Such a structure requires components that are used for unbundling the bundle fiber inside the interface. Thus, the reduction in the size of the interface is limited.